1. Field of the Invention
This invention relates in general to implantable heart-stimulating devices. More particularly, the invention relates to external control over such devices for providing stimulating impulses at a so-called demand-rate, even in the presence of natural heartbeats. More particularly still, the invention relates to the provision of such test modes in such devices to allow at least the determination of the condition (i.e. remaining life) of the implanted power source.
2. Description Of Prior Art
Implantable heart-stimulating devices, of both a continuous and demand type, have been disclosed in prior art. External means for controlling operation of the implanted device has also been disclosed, i.e., externally magnetically operated reed switch. An example of these reed switches can be seen in U.S. Pat. No. 3,311,111 to Bowers. However, the operation of these prior art reed switches do not provide the test mode or modes of the present invention.
In another patent to Bowers, U.S. Pat. No. 3,563,247, there is disclosed an external control for varying rate of stimulation pulses. The patent discloses a variation in pulse rate for the purpose of stimulating the patient at a therapeutic rate different from the initial pacer rate provided. Although the present invention also provides a variation in pulse rate, it is not for providing the patient with a new therapeutic stimulation rate different from the initial pacer rate. The present invention utilizes rate information during a temporary test mode of operation to provide at least an indication of remaining life of the implanted power source. This is not disclosed in the prior art.
Certain pacers on the market today are designed to maintain a stable pulse rate regardless of battery condition. There is no apparent rate change or other indication of forthcoming failure of the device until several cells of the total number of cells have failed. Then catastrophic failure may occur. Although apparent life of the pacer is lengthened, actual life of the pacer is not changed at all.
This approach to implantable pacers is desirable from a marketing point of view, but undesirable from a life-support point of view. The patient's life depends on continued proper functioning of implanted circuitry. The pacer may lose capture (the ability of the pacer to stimulate the heart) because of the reduced pulse energy. With this type of design, there is no readily available way of checking the batteries at various times throughout the pacer's life.
It is thus desirable to know how much battery life remains at varying points in time after implantation. An undesirable way to determine battery life is to utilize a surgical procedure and remove the implanted pacer to test the batteries. This is, of course, a poor approach. It is desirable to make the determination of remaining power source life by observing some characteristic of the pacer while it remains implanted. This characteristic of the pacer should be selected to provide an indication to the observer of the state of the power source. A solution to this battery depletion-sensing problem is provided by utilizing an externally manually controlled test rate mode of operation. In the normal demans mode of operation, the pacer provides a pulse rate substantially independent of power source level.
A pacer having the immediately aforementioned externally manually controlled test rate mode of operation is disclosed in U.S. Pat. No. 3,774,619 to Goldberg. In that patent, an externally located and operated magnetic-field source was utilized to control implanted test mode pacer circuitry arranged to cause dependence of the heart-stimulation rate upon the level of the power source when in the test mode, but arranged to provide a heart-stimulating rate independent of the level of the power source when in the ordinary demand mode. The test mode circuitry included a magnetically operated reed switch, and other circuitry to cause a battery-dependent disproportionate change between the timing-capacitor charging rate and the threshold level of an implanted relaxation oscillator. This made the oscillator frequency and thus the heart-stimulating rate controllably dependent upon power source level.
In order to determine the condition (i.e. the level of depletion or remaining life) of the pacer's batteries, the stimulation rate occurring during the test mode and dependent upon the level of the power source required comparison with a stimulation rate which was independent of the remaining life of the batteries. Although the stimulating impulses occurring at the demand-rate would provide such reference for comparison, those stimulation impulses would not be present if the patient's heart was functioning normally. Accordingly, the 3,774,619 patent discloses using the so-called "interference-continuous" mode for obtaining stimulating impulses which occur at a rate relatively independent of the remaining life of the battery and subsequently subtracting some predetermined number of beats per minute from that rate to obtain the demand rate. While that technique for "computing" the demand-rate is generally satisfactory in instances where the demand-rate is not available, the rate associated with the "interference-continuous" mode may tend to vary somewhat, whereas the demand-rate is inclined to be more stable. Further, in order to create the "interference-continuous" mode of operation, it is necessary that the magnetic field which actuates the reed switch be pulsed at a rate of more than about 15 pulses per second, thereby obviously requiring an external oscillator.
Accordingly, it is a principal object of the present invention to provide an improved implantable heart-stimulating device of the demand type.
It is a further object of the invention to provide an implantable demand heart pacer capable of temporarily generating stimulating impulses at the demand-rate in the presence of natural (spontaneous) heartbeats.
It is an additional object of the invention to provide improved externally controlled implantable test mode circuitry to externally determine depletion of implanted batteries of a demand pacer.
These and other objects will be in part obvious and in part pointed out in greater detail hereinafter.